ORIGINAL RESEARCH PAPER

4D seismic time-lapse monitoring of an active cold vent, northernCascadia margin

Michael Riedel

Received: 28 June 2007 / Accepted: 3 December 2007 / Published online: 5 January 2008

� Springer Science+Business Media B.V. 2007

Abstract Two single-channel seismic (SCS) data sets

collected in 2000 and 2005 were used for a four-dimen-

sional (4D) time-lapse analysis of an active cold vent

(Bullseye Vent). The data set acquired in 2000 serves as a

reference in the applied processing sequence. The 4D

processing sequence utilizes time- and phase-matching,

gain adjustments and shaping filters to transform the 2005

data set so that it is most comparable to the conditions

under which the 2000 data were acquired. The cold vent

is characterized by seismic blanking, which is a result

of the presence of gas hydrate in the subsurface either

within coarser-grained turbidite sands or in fractures, as

well as free gas trapped in these fracture systems. The

area of blanking was defined using the seismic attributes

instantaneous amplitude and similarity. Several areas

were identified where blanking was reduced in 2005

relative to 2000. But most of the centre of Bullseye Vent

and the area around it were seen to be characterized by

intensified blanking in 2005. Tracing these areas of

intensified blanking through the three-dimensional (3D)

seismic volume defined several apparent new flow path-

ways that were not seen in the 2000 data, which are

interpreted as newly generated fractures/faults for upward

fluid migration. Intensified blanking is interpreted as a

result of new formation of gas hydrate in the subsurface

along new fracture pathways. Areas with reduced

blanking may be zones where formerly plugged fractures

that had trapped some free gas may have been opened and

free gas was liberated.

Keywords 4D seismic time-lapse imaging �Seismic processing � Gas hydrate � Cold vent �Fracture systems

Abbreviations

IODP Integrated Ocean Drilling Program

LWD Logging-while-drilling

NEPTUNE Northeast Pacific time-series undersea

networked experiments

RMS Root-mean square

SCS Single channel seismic

TWT Two-way travel time

2D Two-dimensional

3D Three-dimensional

4D Four-dimensional

Introduction

Cold vents are commonly observed on active margin set-

tings where they contribute significantly to the fluid flow

within the accretionary prism. Such active cold vents were

observed for example on the Aleutian margin (Suess et al.

1998), at Hydrate Ridge on the Cascadia margin offshore

Oregon (e.g., Suess et al. 1999, 2001), offshore Vancouver

Island (Riedel et al. 2002, 2006a), at the Nankai Trough

offshore Japan (e.g., Kobayashi 2002 and reference

therein), and along the Makran margin (e.g., von Rad et al.

2000). They are also seen in passive margin settings such

as offshore Nova Scotia (Shimeld et al. 2004), at the Blake

Ridge (Paull et al. 1996) and in the Gulf of Mexico (e.g.,

Sassen et al. 2001 and references therein). Cold vents

are commonly associated with seafloor chemosynthetic

communities (clam colonies and bacterial mats) and with

M. Riedel (&)

Department of Earth and Planetary Sciences, McGill University,

3450 University Street, H3A2A7 Montreal, QC, Canada

e-mail: mriedel@eps.mcgill.ca

123

Mar Geophys Res (2007) 28:355–371

DOI 10.1007/s11001-007-9037-2

wide-spread carbonate formations. Often an association

with near-seafloor gas hydrate is found.

Active cold vents are areas where changes of the sea-

floor and subsurface conditions can change rapidly.

Probably the best studied site is Hydrate Ridge offshore

Oregon where rapid changes of gas venting were observed

(e.g., Tyron et al. 1999; Suess et al. 1999; Collier et al.

1999). Typically active gas venting has been observed at

several individual smaller vent outlets that can switch in

location rapidly within days or weeks (G. Bohrmann,

personal communication). The reason for the change in the

vent outlet location and activity can only speculated upon

as only few long-term monitoring data sets are presently

available. Changes in the subsurface, potentially the cause

for observable changes at the seafloor, have not been

monitored systematically.

The area of this study is located on the northern

Cascadia margin offshore Vancouver Island (Fig. 1) at

Integrated Ocean Drilling Program (IODP) Site U1328.

The vent field is characterized by several seismic blank

zones varying in size from a few tens of meters to several

hundred meters in diameter. Bullseye Vent is the largest

and most prominent vent in this vent field and has been a

site of intense studies over the past years. Studies include

numerous two-dimensional (2D) and three-dimensional

(3D) single and multichannel seismic surveys (Riedel et al.

2002), Ocean-Bottom-Seismometer surveys (Hobro et al.

2005; Spence et al. 1995; Zykov and Chapman 2004),

piston coring with physical property measurements

(Novosel 2002; Novosel et al. 2005) and geochemical

analyses (Solem et al. 2002; Pohlman et al. 2003; Riedel

et al. 2006a), heat flow measurements (Riedel et al.

2006a), bottom-video observations (Riedel 2001; Riedel

et al. 2006a), controlled-source electromagnetic imaging

(Schwalenberg et al. 2005) and seafloor compliance mea-

surements (Willoughby et al. 2005). Bullseye Vent was

also a target of the most recent IODP Expedition 311

during which five holes were drilled at Site U1328 along a

100 m section through the centre of the vent (Riedel et al.

2006b).

This paper presents an attempt at four-dimensional (4D)

seismic time-lapse imaging of an active cold vent. This

paper has two goals: (a) to demonstrate the technique of

how to process challenging unconventional single-channel

seismic (SCS) data sets for 4D time-lapse monitoring, and

(b) to demonstrate that changes did indeed occur within the

cold vent over the period of 5 years (2000–2005) and can

be mapped accurately.

Detecting temporal changes in the subsurface using

geophysical imaging techniques is by now routine in the

oil-and-gas industry (e.g., Lumley 2001 and references

therein). A standard seismic processing technique in

4D time-lapse monitoring is known as cross-equalization.

The main purpose of 4D seismic cross-equalization is to

reduce differences in areas with no expected changes and

to optimize 4D difference anomalies. Examples of using

seismic cross-equalization in 4D imaging can for example

be found in Eastwood et al. (1998), Harris and Henry

(1998), Naess (2006), and Gan et al. (2004).

The cross-equalization technique was adopted in this

study to compare two SCS data sets acquired over Bullseye

Vent where the location and nature of subsurface changes

(if at all present) are not previously known. The two data

sets were collected with similar acquisition parameters and

geometries, but substantial systematic differences were

observed between them. An enhanced seismic imaging

Fig. 1 (a) Map of general location of survey area on the northern

Cascadia margin. The zone of regional gas hydrate occurrence is

shown in gray inferred from the occurrence of bottom-simulating

reflections in regional seismic data. (b) Detailed map of the area of

the cold vent field near Ocean Drilling Program Site 889, IODP Site

U1327 and U1328. (c) Detailed outline of Bullseye Vent and location

of piston cores and IODP Site U1328 drill holes. The shaded outline is

the area of active venting, characterized by seismic blanking

356 Mar Geophys Res (2007) 28:355–371

123

technique for defining areas affected by fluid flow and/or

enhanced gas hydrate concentrations by using 3D geo-

metrical seismic attributes such as similarity (also referred

to as seismic coherency) is introduced. A potential limi-

tation of this time-lapse analysis of SCS data may be that it

is more sensitive to noise, which is not attenuated with

stacking. High frequency signals yield better resolution for

small changes of reflectors, but the use of high frequencies

demands an efficient noise removal filter. In this case study

where relatively high frequency signals were used, but

noise was difficult to attenuate, we introduce seismic

attributes that may help overcome this issue (i.e., similar-

ity) and limit our interpretation to only large-scale

variations that can be observed consistently across the 3D

seismic data.

The results of this 4D time-lapse imaging showed that

even relatively low-quality 3D seismic reflection data can

be used to detect subsurface changes if the seismic data is

processed appropriately. However, significantly better

results can be expected if for example the seismic receivers

are permanently placed on the ocean floor, which not only

reduces navigation uncertainties, but also increases the

signal-to-noise ratio. The study presented here shows that

subsurface changes in the cold vent are to be expected to be

detectable over timescales of a few years or less with

repeat-seismic surveys, and can be imaged using advanced

4D time-lapse processing and imaging techniques. This

result has important implications for the application of 4D

seismic imaging using the NEPTUNE cable observatory

that is currently implemented on the Cascadia margin.

Summary of Bullseye Vent observations

Bullseye Vent is part of a larger cold vent field that covers

an area of about 2 9 4 km on the mid slope of the northern

Cascadia margin (Fig. 1). Four main vents were identified

in this vent field and vary in size from a few tens of meters

to several hundreds of meters in diameter. All vents are

seismically characterized by reduced reflection amplitude

or blanking.

Massive pieces of gas hydrate were recovered by piston

coring (maximum 8 m penetration) at Bullseye Vent as

near as 0.5 m below the seafloor (Riedel et al. 2006a).

Logging-while-drilling (LWD) measurements at IODP Site

U1328 showed a zone of very large electrical resistivity of

up to 30 X m within the top 40–50 m below seafloor

(mbsf) in the centre of the vent (Riedel et al. 2006b). IODP

coring at this site also recovered several massive chunks of

gas hydrate within the top 50 mbsf, but gas hydrate was

only sporadically recovered at greater depth. LWD data at

IODP Site U1328 imaged several steeply dipping fractures

of very high electrical resistivity representing high gas

hydrate concentration within the fracture (Riedel et al.

2006b).

The controlled-source electromagnetic data also showed

large resistivity anomalies across all vents, with electrical

resistivities as high as 6 X m, which is around five times

the assumed regional no-gas hydrate background, sug-

gesting the presence of high gas hydrate concentrations at

those vents (Schwalenberg et al. 2005). Additional con-

straints on the subsurface plumbing were derived from

several seafloor compliance measurements at and around

Bullseye Vent. The compliance data show an overall

increase in shear-modulus (and hence shear-wave velocity)

below the vent within the depth range of the gas hydrate

stability zone, whereas no such pronounced shear-modulus

anomaly was seen away from the vent (Willoughby et al.

2005). The increase in shear-wave velocity is interpreted as

additional evidence for the presence of gas hydrate within

the cold vent. However, the compliance data do not provide

fine resolution of the depth distribution of gas hydrate.

Repeated seafloor video observations in 2000 and 2001

showed that the seafloor around Bullseye Vent is charac-

terized by wide-spread carbonate platforms and nodules, as

well as isolated chemosynthetic communities (Riedel 2001;

Riedel et al. 2006a; Beaudet et al. 2001). Water samples

collected over an active chemosynthetic community in

2001 showed evidence of methane released into the water

column up to a height of 200 m above the seafloor (Solem

et al. 2002). Surveys conducted in September 2006 using a

12 kHz echosounder, mapped extensive gas discharge

across Bullseye Vent where the plume structures rise to

*500 m below sea surface.

Measurements of physical properties of the top 8 m of

sediments within and around Bullseye Vent showed further

evidence of a long-time gas flux history. Magnetic sus-

ceptibility is reduced by several orders of magnitude inside

Bullseye Vent as result of chemical reduction of magnetic

minerals to pyrite through enhanced methane flux (Novosel

et al. 2005). Geochemical pore-water analyses further

demonstrated that the sulfate–methane interface is much

shallower inside Bullseye Vent than outside. Near the area

of the active chemosynthetic community the SMI is right at

the seafloor (Pohlman et al. 2003).

A model for seismic blanking

The nature of the seismic blanking has been highly debated

in the literature (Lee and Dillon 2001; Riedel 2001; Riedel

et al. 2002; Wood et al. 2002; Zuhlsdorff and Spiess 2004).

A discussion and comparison of these different models

was given by Riedel et al. (2006a). Their integrated model

appears most closely to the combined geophysical imaging

results and the IODP drilling. This model describes

Mar Geophys Res (2007) 28:355–371 357

123

Bullseye Vent as a complex subsurface network of frac-

tures partially filled with gas hydrate, feeding methane to

the near-surface where massive gas hydrate can form as

methane solubility drastically decreases. Seismic blanking

(seen as vertical wipe-outs) is therein a result of a combi-

nation of transmission losses at the massive gas hydrate cap

and subsurface gas hydrate accumulations (as found in

fractures). Laterally extensive blanking can occur by the

preferential gas hydrate accumulation in the coarser-

grained turbidite layers following the model by Lee and

Dillon (2001). The frequency dependent nature of the

blanking described by Riedel et al. (2002) is related to the

magnitude of seismic waves being affected by these het-

erogeneities in the subsurface. Lower seismic frequencies

(and thus larger wavelengths) are able to penetrate deeper

as they are less affected by small-scale heterogeneities (and

thus will be less attenuated) than higher frequencies with

smaller wavelengths.

4D time-lapse processing sequence

In this section the processing sequence used to match the

two data sets is described in detail. Although 4D seismic

imaging is by now established in the oil-and-gas industry,

it has not yet widely been used in academic science.

Furthermore, the unusual nature of the data sets (with

respect to standard oil-and-gas industry) and their appli-

cation to a cold vent environment, both require additional

care to ensure that no artifacts are introduced resulting in

false interpretation of the data.

The seismic processing steps for the complete 4D seis-

mic analysis and comparison are summarized in a flow

chart in Fig. 2. The two SCS data sets were not migrated

for this analysis because only sparse reliable velocity

information is available and migration with inaccurate

velocities would result in degraded imaging results. Prior

to any 4D processing and interpretation, both data sets

were simply band-pass filtered to remove some of the high-

frequency noise (cutoff was 300 Hz).

Survey geometry, and 3D binning

In 2000 and 2005, two sets of parallel 2D SCS lines were

acquired for 3D imaging of Bullseye Vent (Fig. 3). Each

set consists of 21 lines spaced roughly 25 m apart. The

streamer and airgun were deployed in both surveys in a

similar fashion resulting in similar parameters for cal-

culating the common mid-point locations. In both

surveys a 40 in3 (0.75 l) airgun with a wave-shape

kit that helps reducing the bubble pulse was used. The

airgun is towed at a nominal depth of 2 m below the

sea-surface. The airgun generates seismic signals over a

broad spectrum ranging from 20 to 500 Hz with a central

frequency of *120 Hz (Fig. 4). The Teledyne seismic

streamer consisted of a 25 m active section hosting 50

hydrophones each 0.5 m apart. The oil-filled streamer

generally settles at a depth of *4 m below the sea sur-

face (Riedel 2001).

The 2000 data were merged into a rectangular 3D

volume by binning the data using an inline spacing of 25 m

and a crossline spacing of 12 m. The 2005 data had

irregular shot point spacing across the survey area and also

showed significant line orientation divergence. The data

were therefore binned with a coarser bin size of 30 m in

the inline direction and 18 m in the crossline direction. The

first step in the processing sequence is re-binning of the

2005 survey and interpolation. The 2000 data served as

reference and the 2005 data were forced onto a 25 9 12 m

grid using the four nearest neighboring traces from the

2005 survey and linear interpolation.

Time- and phase-shifting

The next processing step after re-binning is a time- and

phase-matching of the 2005 data. The data sets were flat-

tened to a common datum at 1.6 s two-way travel time

(TWT) using the seafloor reflection. However, some small

vertical time offsets were still present after datuming and

need to be removed. The phase-matching is carried out by

Fig. 2 Flow chart illustrating the 4D seismic processing sequence

358 Mar Geophys Res (2007) 28:355–371

123

calculating the cross-correlation between coincident traces

from both data sets and identifying the time and phase shift

required to maximize the match between both traces over a

specifically defined window length. Since it is generally not

known where and to what magnitude changes did occur, a

horizon needs to be identified where no changes have

occurred between the two surveys. In this study the seafloor

serves as such a reference horizon. The cross-correlation

window for the time- and phase-matching was centered at

1.6 s TWT with a 10 ms running window above and below

this datum.

Gain adjustment

Both data sets were acquired with different overall gain

settings in the field. The individual 2D lines also show

amplitude variations that result in striations in time-slices.

These amplitude variations within the 2000 data were

removed prior to the 4D analyses by balancing all 2000

data relative to a reference amplitude, which was arbitrarily

chosen as the root-mean-square (RMS) amplitude of

inline 1. To remove the gain difference between the 2005

and balanced 2000 data, a gain adjustment was carried out

that calculates the RMS amplitude of all traces in the 2000

data and compares it to that of all traces in the 2005 data.

A 1 s TWT time window spanning the entire data range

(1.5–2.5 s TWT) was used to define the RMS amplitudes

and a single, global scalar is used for all traces to adjust the

2005 data.

Fig. 4 (a) Waveform and (b) frequency spectra of the original 2000

seismic data (blue), original 2005 data (red) and 2005 data with 4D

processing applied (green)

Fig. 3 Map of survey line geometry and 3D cube outline of the (a) 2000 reference data and (b) 2005 data

Mar Geophys Res (2007) 28:355–371 359

123

Shaping filter

The last step in the processing sequence is defining a

shaping filter that transforms the 2005 data to match the

2000 data for a specific horizon. The idea behind this step

is to reproduce a section out of the 2005 data that is as

close as possible to the 2000 data acquisition parameters

(source wave-form and frequency content), but which

preserves any real changes present in the sub-surface. The

flattened seafloor was again used as a reference horizon

where no changes were believed to have occurred between

the two surveys. A 20 ms long window centered on the

seafloor at 1.6 s TWT was chosen to define the shaping

filter. The result of this shaping filter is that the seafloor

reflection defined over the 20 ms long window is identical

in both surveys.

Evaluation of 4D processing results

The effect of the 4D seismic processing steps needs to be

carefully evaluated in order to avoid over-interpretation of

the differences in the 2000 and 2005 data. The first pro-

cessing step involved re-binning by trace interpolation of

the 2005 data to match survey geometries. To evaluate this

process, time-slices of the flattened 3D volumes at 1.65 and

1.68 s TWT for the reference data and the 2005 data are

compared with time-slices of the flattened data prior to

merging them into a 3D volume using the original common

mid point locations (Figs. 5 and 6). The comparison shows

that the same large-scale structural features are present in

both data prior to (Figs. 5a, b and 6a, b) and after binning

(Figs. 5c and 7c) and that the re-binning and interpolation

of the 2005 data did not introduce any false structural

information. The seismic amplitude slice at 1.65 TWT

(Fig. 5c–e) and the instantaneous amplitude or envelope

(Fig. 5f–h) clearly show that the 4D process was successful

in generating a 3D data volume out of the original 2005

data that is much closer in the overall characteristics of the

2000 data. In the envelope slices at 1.65 s TWT there are

two structural elements very well preserved in the data

after 4D processing that are very similar to those seen in

the original 2000 data: (1) Two parallel low-amplitude

lines with a high-amplitude centre are seen in the SW lower

half of the data cube. This feature is not evident in the 2005

data prior to the processing, but is again visible afterwards.

(2) In the Northeast upper corner a high-amplitude ring-

like structure is seen around a prominent blank zone.

Although this feature is recognizable in the original 2005

data, the feature is more clearly defined after applying the

4D processing sequence. The comparison of the slice at

1.65 s TWT also shows the difficulties of defining mean-

ingful differences over the centre of Bullseye Vent, where

the slice after 4D processing (Fig. 5h) shows prominent

high-amplitude spots not present in the 2000 data set.

Similar observations can be made on the time-slice at

1.68 s TWT (Fig. 6c–h). Amplitudes after 4D processing

appear more similar to the 2000 reference data and the

same 3D structures are preserved, but some significant

changes can be identified, which are described in more

detail below.

The comparison also shows that the 4D processing

sequence fails over areas with steep dips, or within areas of

accreted sediments, especially towards the southwest and

northeast corners of the 3D seismic cubes. While the ori-

ginal 2000 data show some coherent reflectivity, the

structures are not well re-produced after the 4D processing

of the 2005 data set. This is certainly partially a result of

the relative poor input data of the 2005 survey, but it also

defines the limit over which 4D time-lapse analyses should

be attempted. If the sub-bottom reflections are seafloor- to

sub-seafloor-parallel there is a high chance of succeeding

in generating meaningful data for comparison. If the

structures are too steep, the comparison becomes less

reliable. Acquiring data with higher inline and crossline

spacing or multi-fold data can help to push the limit of

structural dip that can be included in the analysis. In this

study interpretations are limited to areas with dips less than

8 � (equivalent to *2 ms TWT depth change per trace, or

*2 m depth change over a horizontal distance of 12 m

using an average velocity of 1,600 m/s in the upper 100 m

below seafloor). This limitation excludes about one-third of

the area covered by the 3D volumes, or about 300 m along

an inline on the Northeast and Southwest corner.

Evaluation of the time- and phase-matching, as well as

shaping filters, is best carried out on sections of the actual

seismic inlines. Inlines 9, 12 and 19 were chosen to

demonstrate the effects (Figs. 7–9). In the case of inline 9

the apparent blanking seen around crossline 70 in the

original 2005 data (Fig. 7b) was entirely the result of poor

airgun shots, as seen in the deteriorated seafloor reflec-

tion. The apparent blanking has been removed and

amplitudes were restored by the 4D processing. Inline 12

was chosen for this comparison because it is located over

the centre of Bullseye Vent and crosses the massive gas

hydrate cap previously identified (Riedel et al. 2002).

This cap is clearly shown in the original 2000 data

between crosslines 50 and 85 within the top 20 ms below

seafloor (Fig. 8a). The re-binned 2005 data prior to the 4D

processing do not show as strong evidence of this cap

reflection but similar subsurface blanking is apparent

(Fig. 8b). However, some of the blanking around the cap

reflector appears to be not as prominent after applying the

4D processing sequence, whereas two smaller-scale

blanking areas around crosslines 30 and 50 are still

present (Fig. 8d). This line also shows the complex nature

360 Mar Geophys Res (2007) 28:355–371

123

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362 Mar Geophys Res (2007) 28:355–371

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of the centre of the cold vent around the massive gas

hydrate cap. The artificial vertical shifts (specifically at

crossline 78 and 82) in the 2005 data after 4D processing

are a result of the irregular seafloor reflection around the

cap and the chosen window length for the cross-correla-

tion to define the shaping-filter. Any changes observed on

time-slices around the centre of the blank zone should

therefore be treated cautiously.

Fig. 7 Comparison of seafloor-flattened seismic sections of inline 9

from Bullseye Vent data sets. Shown are seismic amplitudes. (a)

Reference data from 2000, (b) re-binned 2005 data (no processing

applied), (c) re-binned data of 2005 with time- and phase-shifting and

overall gain adjustment applied, (d) re-binned 2005 data with

complete 4D processing sequence applied

Mar Geophys Res (2007) 28:355–371 363

123

Inline 19 showed highly variable seafloor waveforms

between crosslines 10 and 50 in the original 2005 data

(Fig. 9b) and apparent vertical subsurface blanking.

However, after complete 4D processing, this artificial

blanking was removed (Fig. 9d). Further along the same

line, between crosslines 70 and 90, the original 2005 data

showed wide-spread subsurface blanking, which is not

vertically constrained compared to the blanking seen

Fig. 8 Comparison of seafloor-flattened seismic sections of inline 12

over centre of Bullseye Vent with gas hydrate cap-reflector. Shown

are seismic amplitudes (negative amplitude in red, positive amplitude

in black). (a) Reference data from 2000, (b) re-binned 2005 data (no

processing applied), (c) re-binned data of 2005 with time- and phase-

shifting and overall gain adjustment applied, (d) re-binned 2005 data

with complete 4D processing sequence applied

364 Mar Geophys Res (2007) 28:355–371

123

between crosslines 10 and 50 (Fig. 9b). This blanking

was preserved by the 4D processing (Fig. 9d) and is not

seen in the 2000 reference data (Fig. 8a). It is therefore

interpreted as a real geologic signal and evidence for

temporal changes in the subsurface blanking of Bullseye

Vent.

Fig. 9 Comparison of seafloor-flattened seismic sections of inline 19

from Bullseye Vent data sets. Shown are seismic amplitudes (negative

amplitude in red, positive amplitude in black). (a) Reference

data from 2000, (b) re-binned 2005 data (no processing applied),

(c) re-binned data of 2005 with time- and phase-shifting and overall

gain adjustment applied, (d) re-binned 2005 data with complete 4D

processing sequence applied. Details see text

Mar Geophys Res (2007) 28:355–371 365

123

Mapping changes in subsurface blanking using seismic

attributes

The aim of this section is to demonstrate what seismic

attributes are best suited for showing differences in the sub-

surface properties, as well as to show that significant changes

did occur over the time span between the two surveys.

Seismic blanking is linked to the presence of gas hydrate and/

or free gas, as outlined above. Changes in the area affected by

blanking indicate that the amount and/or location of gas

hydrate (and/or free gas) have changed. Mapping these

changes accurately is the first step in developing a geologic

scenario that may gave rise to those changes.

Seismic blanking was previously identified for the cold

vents on the northern Cascadia margin and mapping using

time-slices of instantaneous amplitude showed character-

istic high-amplitude rims around the blank zones for all

four main cold vents identified (Riedel 2001; Riedel et al.

2002; Wood et al. 2000). Those amplitude rims were

interpreted as the result of constructive interference of

diffractions from the top of the gas hydrate cap with dee-

per, regular reflectivity (Riedel 2001; Riedel et al. 2002).

In the case of Bullseye Vent, a prominent ring structure

was identified surrounding the central blank zone. The

central blank zone is outlined in all time-slices shown in

Figs. 5 and 6. Blanking that is easily identified on vertical

seismic sections displaying regular seismic amplitude with

phase information preserved, is more difficult to follow on

time-slices showing amplitudes of arbitrary phases (see

e.g., Figs. 5a–c and 6a–c). Removing phase information by

calculating the seismic envelope (instantaneous amplitude)

shows a much clearer image of the area affected by

blanking (Figs. 5f–h and 6f–h).

The geometrical attribute ‘‘similarity’’ or ‘‘coherence’’

(Taner 2000 and references therein) appears to generate

clearer images of blanking and is thus used in mapping

those areas. This attribute is computed over a specific

window size, which is larger than the length of the domi-

nant wavelength of the data. It identifies the overall

similarity of a trace and its nearest neighbors. For the

Bullseye Vent data the similarity attribute is computed over

a frequency range from 20 to 160 Hz, a window length of

10 ms and incorporates the nearest four neighboring traces.

The time-slice of the similarity attribute at 1.65 s TWT

of the reference 2000 data shows several clear areas of

reduced seismic similarity (Fig. 10a) that are not as easily

Fig. 10 Time-slices of similarity attribute 1.65 s TWT using (a)

2000 reference data (balanced), (b) 2005 data with 4D processing

applied. Darker colors represent higher similarity. (c) Time-slice of

difference between similarity shown in (a) and (b) of the two surveys.

The 2005 data were subtracted from the 2000 data. Red colors show

positive difference (enhanced blanking, less similarity in 2005), black

colors are negative difference (reduced blanking, higher similarity in

2005), (d) line drawing outlining main features (labeled 1 and 2)

compared in study; for details see text. North arrow is shown in (d).

The color scales for the similarity attributes in (a) and (b) are between

0 (white) and 1.5 (black); the color scale for the difference plot in (c)

is between -0.225 (black) and +0.225 (red). IODP Site U1328 drill

holes are shown as black dots. The locations for inlines 9, 12, and 19

as shown in Figs. 7–9 are indicated by the dashed yellow lines

366 Mar Geophys Res (2007) 28:355–371

123

identified from the slice of instantaneous amplitude

(Fig. 5f). The centre of Bullseye Vent covers an area of

400 m (in the Southwest–Northeast direction) by 270 m (in

the SE–NW orientation). Towards the northeast two addi-

tional smaller blank areas surrounded by rims of high

similarity are identified. The traces of two low-similarity

lineaments earlier identified on time-slices of instantaneous

amplitude are also clearly visible in the lower southwestern

half of the similarity time-slice. The time-slice at the same

depth generated from the 2005 data with 4D processing

applied shows an overall similar structure. A strikingly

similar element is the pair of low-similarity traces in the

lower southwestern half of the cube.

The two smaller-sized blank areas to the northeast of the

centre of Bullseye Vent are preserved, although the mag-

nitude of similarity has changed compared to 2000. This is

best visualized on a time-slice of the difference in simi-

larity (Fig. 10c). The color code used represents positive

differences in red and negative difference in black. Since

the difference was calculated by subtracting the similarity

of the 2005 processed data from the 2000 reference data,

red areas identify zones where similarity is lower in 2005

than in 2000, i.e., the traces show less coherent reflectivity

in 2005 than in 2000. Similarly areas in black identify

zones where similarity in the 2005 data is larger than in

2000 reference. The red areas are zones where blanking

intensified since 2000 and black areas are zones of reduced

blanking.

On the time-slice at 1.65 s TWT the area labeled ‘‘1’’ in

the Northeast corner is a blank area that has ‘‘healed’’ since

2005 as blanking appears to be reduced (Fig. 10d). This

can also been seen on inline 12 (Fig. 8a, d). The centre of

Bullseye Vent is dominated by prominent red colors

(labeled ‘‘2’’) in the difference plot (Fig. 10c, d) showing

an overall increase in blanking. The time-slice of similarity

difference at 1.68 s TWT shows almost entirely red colors

(labeled ‘‘2’’) over the centre and surrounding areas of

Bullseye Vent (Fig. 11). Blanking appears to have not only

been intensified, but also spread to a larger area. The same

small blank zone labeled ‘‘1’’ that showed apparent healing

at shallower depth is completely absent at this depth in the

2005 data, whereas it can still be identified in the 2000

data. An area (East–West trending) with an apparent

increase in similarity dominates the southwestern part of

Fig. 11 Time-slices of similarity attribute 1.68 s TWT using (a)

2000 reference data (balanced), (b) 2005 data with 4D processing

applied. Darker colors represent higher similarity. (c) Difference

between similarity shown in (a) and (b) of the two surveys. The 2005

data were subtracted from the 2000 data. Red colors show positive

difference (enhanced blanking, less similarity in 2005), black colors

are negative difference (reduced blanking, higher similarity in 2005),

(d) line drawing outlining main features (labeled 1, 2 and 3)

compared in study; for details see text. North arrow is shown in (d).

The color scales for the similarity attributes in (a) and (b) are between

0 (white) and 1.5 (black); the color scale for the difference plot in (c)

is between -0.225 (black) and +0.225 (red). IODP Site U1328 drill

holes are shown as black dots. The locations for inlines 9, 12, and 19

as shown in Figs. 7–9 are indicated by the dashed yellow lines

Mar Geophys Res (2007) 28:355–371 367

123

Bullseye Vent at this depth (labeled ‘‘3’’ (Fig. 11); how-

ever, this change in similarity is not as prominent as other

changes identified between the surveys.

Interpretation of changes in similarity on individual

time-slices may be confusing and not inherently convinc-

ing. However, when zones of increased seismic similarity

identified on Figs. 10 and 11 are traced from time-slice to

time-slice, they form downward continuations, resembling

somewhat the nature of fracture zones (Fig. 12). In Fig. 12

the similarity of the 2000 data along inline 17 is compared

to the similarity after 4D processing. The difference plot

highlights the changes in similarity (red colors indicate

increased blanking in 2005, i.e., reduced similarity). Most

of the changes resemble the form of semi-vertical fracture

zones. There are also two zones showing horizontally

extended, or ‘‘bedded’’-like, areas of reduced similarity in

2005 between crossline 50 and 80, at 1.68 and 1.78 s TWT,

respectively. It should be emphasized that this interpreta-

tion is still somewhat speculative in nature and needs

verification by additional 4D seismic imaging.

Discussion and significance of observed changes

Although changes in the seismic amplitudes and related

attributes (especially similarity) of the two data sets were

identified, significant uncertainties in their meaning

remain. Are these meaningful results or artifacts of the 4D

processing sequence?

The seismic processing sequence applied tried to

remove differences that result from the acquisition induced

variations of source signature and frequency content. The

re-binning of the 2005 data to match the same 3D geometry

of the 2000 data has not introduced any false structural

elements and is believed to be adequate for the purpose of

comparing the two data sets with the constraint of dips of

less than 8�. Using the similarity attribute to define areas

affected by blanking is more robust than the actual

amplitude itself because it removes phase information. The

similarity attribute was further calculated over a 10 ms

long window and frequencies were limited to 160 Hz,

which results in an overall smoother image, leaving only

larger-scale features within the data. Thus the process is

less affected by high-frequency noise.

The 4D processing sequence relies on the assumption

that the seafloor is unchanged between the two surveys and

that the trace-by-trace shaping filter can be applied and

does not introduce artifacts. However, if changes did occur

in the near-seafloor range (e.g., within the centre of the

vent) covered by the filter length, then those changes would

be propagated into the subsurface and subsequently inter-

preted as deep-rooted fluid-flow related changes. To

overcome this problem, only seismic data away from the

centre and gas hydrate cap could be utilized to create

global shaping filter parameters; however, the data sets

used in this study cover too small an area to be significantly

distant from the centre of Bullseye Vent. Thus there is a

remaining uncertainty for the origin of apparent changes

below the centre of the vent and any mapped changes

should be treated with caution.

If the observed changes are believed to be real, what

caused the change in blanking? Several differing models

were brought forward to explain the nature of Bullseye

Vent and its associated seismic blanking (Riedel et al. 2002,

2006a; Wood et al. 2002; Zuhlsdorff and Spiess 2004).

As described above, the model by Riedel et al. (2006a)

Fig. 12 Comparison of seismic similarity attribute of inline 17. (a)

Similarity from the 2000 data set, (b) similarity from 2005 data after

4D processing applied, (c) difference in similarity showing apparent

new fractures zones and semi-horizontal beds of reduced similarity in

2005. The color scales for the similarity attributes in (a) and (b) are

between 0 (white) and 1.5 (black); the color scale for the difference

plot in (c) is between -0.225 (black) and +0.225 (red). The locations

for inlines 9, 12, and 19 as shown in Figs. 7–9 are indicated by the

dashed yellow lines

368 Mar Geophys Res (2007) 28:355–371

123

describes Bullseye Vent as a network of gas hydrate-filled

fractures with a cap of massive gas hydrate in the top 40–

50 mbsf. Small-scale vent outlets of only a few square

meters in size were observed at the seafloor where tempo-

rarily methane gas can escape through the gas hydrate

stability zone into the ocean. As gas migrates upward

through the network of fractures, it partially transforms into

gas hydrate, potentially blocking some of the fractures and

sealing them for further gas transport. The naturally buoy-

ant gas, which cannot combine with surrounding water to

form gas hydrate, needs to find new pathways, potentially

opening new fractures. Additional gas hydrate is likely to be

formed along those new pathways. Thus, blanking as a

result of scattering at fractures and potentially from the

presence of free gas can change over time. Although most

of the changes in similarity resemble semi-vertical fracture

paths, several zones of reduced similarity in 2005 were seen

that are ‘‘bedded’’-like or semi-horizontal. Those horizontal

extents of reduced similarity could be related to horizontal

migration of methane gas along more porous (sandy)

turbidite layers, attracting gas hydrate formation relative to

the surrounding mud. Several of those turbidite layers filled

with gas hydrate embedded in gas hydrate free mud were

recovered during IODP Expedition 311 (Riedel et al.

2006b). Increased blanking from those hydrate-filled

turbidites can be explained by the model of Lee and Dillon

(2001), where the seismic reflectivity between turbidite

layers and regular mud-dominated sequences is reduced

by the preferential accumulation of gas hydrate in the tur-

bidite layer.

It can further be speculated that gas migration can be

initiated and intensified by earthquake activity. Gas accu-

mulates below the gas hydrate stability zone at the bottom

simulating reflector, or within the cold vent where it is

sealed off from free water and could be rapidly released as

earthquake shaking, helps to open fractures and generate

new pathways.

Future opportunities

As part of the NEPTUNE program it is planned to place

broad-band seismometers near Bullseye Vent recording

earthquake activity. This will be complemented with sea-

floor-based video observations and long-time geochemical

monitoring of the fluid flux. For the second phase of the

IODP Expedition 311 it is proposed to install special

borehole monitoring instruments, including fiber-optic

temperature sensors and fluid samplers. Ideally a set of

permanent seismic receivers should be placed across

Bullseye Vent, allowing for simple and fast acquisition of

4D time-lapse seismic data. With receivers permanently

implemented on the seafloor many of the above described

uncertainties in the 4D time-lapse results could be

removed. The seismic source should also be towed at

greater water depth to remove source-signature variations

from weather induced sea surface conditions. However, the

source should be placed at such a depth that interference

between the primary reflections and the sea-surface ghost

are avoided.

Summary and conclusion

Two 3D single channel seismic reflection data sets were

used for a 4D time-lapse analysis of an active cold vent

(Bullseye Vent). The required processing steps are descri-

bed and imaging techniques are introduced to help best

identify areas of seismic blanking. The data set acquired in

2000 has superior navigation accuracy and serves as a

reference in the applied processing sequence. The 2005

data was re-binned to achieve identical 3D geometries for

the data sets and was subsequently processed using time-

and phase-matching, amplitude adjustment and shape-

filtering. The phase- and shape filters were generated using

the seafloor reflection as reference horizon where no

changes were expected. All seismic data were also flattened

using the seafloor reflection to a common datum (1.6 s

TWT). The 4D processing sequence yielded a data set from

the 2005 data that is most comparable to the conditions

under which the 2000 data were acquired. The area of

blanking (indicative of the presence of gas hydrate and/or

free gas) was defined using seismic attributes such as

instantaneous amplitude and similarity. The seismic simi-

larity attribute of the 4D processed data was subtracted

from the reference similarity volume and was used to

identify areas of apparent change.

The centre of Bullseye Vent and an area around the

centre was seen to be characterized by intensified blanking.

Tracing changes from time-slice to time-slice allowed the

definition of new pathways that are interpreted as newly

formed fractures/faults that allow upward migration of

fluids. Bullseye Vent has previously been characterized by

a subsurface network of fractures that are partially gas

hydrate filled feeding methane gas to shallow depths. Gas

hydrate is also occurring preferentially in coarser-grained

turbidite layers. Seismic reflectivity between turbidite

layers and regular mud-dominated sequences can be

reduced by the presence of gas hydrate in the turbidite

layer. Thus blanking reflects the presence of gas hydrate

and/or free gas in either fractures or sandy layers. Vent

outlets with chemosynthetic communities and periodic gas

escapes that were identified by bottom-video observations

are the seafloor expression of such complicated subsurface

networks. Upward-migrating gas (potentially intensified by

earthquake activity) can open new fracture pathways,

Mar Geophys Res (2007) 28:355–371 369

123

resulting in a switch of the active vent outlet at the surface

and also a change of the area affected by blanking in the

subsurface. The mapped changes between the 2000 and

2005 data set may indicate that new pathways for upward-

migrating methane gas were generated and that gas hydrate

was newly formed in areas with increased blanking. Areas

where blanking was reduced between the two surveys may

reflect areas where formerly trapped free gas in fractures

(disconnected to surrounding water to form gas hydrate)

may have been liberated and moved along new fractures

resulting in the overall ‘‘healing’’ of the blank area.

The results of this 4D time-lapse imaging showed that

even relatively low-quality 3D SCS data can be used to

detect subsurface changes if the seismic data is processed

appropriately.

Acknowledgements The author would like to acknowledge the

important contributions of the Coast Guard crews onboard the

research vessel John P. Tully and scientists involved in the data

acquisition of the two data sets, especially George Spence and Ele

Willoughby. Furthermore the author wants to acknowledge Seismic

Micro Technology for the use of Kingdom Suite and Hampson &

Russell for the use of the program PRO4D used in this analysis.

Additional thanks go to Gilles Bellefleur, Mathieu Duchesne, and Ele

Willoughby for many helpful suggestions, discussions and encour-

agements to carry out this study.

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